The invention relates to mobile communications systems and particularly to a method and equipment for identifying a logical channel in a radio frame part which may comprise information of one or more logical channels, channel decoding of the information being possible by means of channel decoding methods relating to the different logical channels.
In digital radio systems a physical channel provides a link at the interface between a subscriber terminal and a network. A physical channel substantially comprises a frame part of a selected multiple access technique, the frame part being allocated to data transmission between a particular subscriber terminal and the network. A physical channel can therefore comprise for instance one or more TDMA frame time slots arranged at a specific frequency range, or frame parts separated by means of a CDMA frame code.
Physical channels are utilized by means of various multiplexing techniques whereby logical channels are created on a physical link. The term logical channel refers to a logical data transmission bus between two or more parties, the bus being mapped on an interface between a protocol and a radio system. A mobile communications system or a part thereof can therefore comprise different types of logical channels. Logical channels are typically divided into traffic channels (TCH), which comprise different kinds of traffic relaying channels, and control channels (CCH), which comprise e.g. broadcast control channels, common control channels and dedicated control channels. Speech and circuit-switched data are transferred over the radio interface substantially on traffic channels and signalling and packet data on control channels.
As a rule, a logical channel associated with a received signal can be concluded from the used multiplexing technique, but this does not always apply. A number of mobile communications systems allow signalling, for example, to be also transferred on traffic channels when necessary, a burst to be transmitted then preferably comprising information indicating whether transmission of traffic data or signalling is concerned. This procedure will be hereinafter referred to as stealing.
In the TETRA (TErrestrial, Trunked RAdio) digital mobile communications system a physical channel is substantially comprised of one time slot of a TDMA frame comprising four time slots, the time slot corresponding to one burst transferred over a radio path. A normal uplink or downlink time slot typically comprises two blocks, with a bit map called a training sequence between them. A training sequence is used for indicating features relating to transmission timing and distortion, which are typically important in demodulation, to a receiver.
In the TETRA system two normal 22-bit training sequences differing from one another are defined, the training sequences being used for indicating whether the burst blocks comprise one or two logical channels. The above described stealing from a traffic channel is also indicated by using a training sequence. When a burst comprises a training sequence 1 (TS1), stealing is interpreted not to be in use, and the burst comprises entirely traffic channel data. When a burst comprises a training sequence 2 (TS2), the time slot into which the burst is mapped is interpreted to be either totally or partly stolen for signalling purposes.
In circumstances where reception is subject to fading and noise, it has proved to be most difficult to distinguish training sequences, and thus logical channels relating to a time slot, from one another. If a training sequence TS1 is by mistake interpreted as a training sequence TR2, the receiver concludes that a signalling message is concerned, which causes traffic channel blocks to be lost and decreases data transmission capacity. If a training sequence TS2 is by mistake interpreted as a training sequence TR1, the receiver interprets that traffic channel data is concerned, the transmitted signalling being thereby lost. The possibility that a logical channel can be misinterpreted in this way is most disadvantageous for the operation of the system. Errors in interpretations are particularly problematic in channels in which bit error ratio should be very small to support efficient channel coding.